Ranger's focus

I feel like there is a confusing situation with Ashe. So Ashe has her ranger's focus, where attacking will stack it up to 4 times. If her ranger's focus is fully stacked, she can use it on turrets, but she can't stack it up on the turret itself. While attacking a turret with Ashe, her ranger's focus won't stack, but the stacks you already have won't fall off. So for example, if Ashe were to attack one minion and then immediately start attacking a turret, her ranger's focus would stay at 1 stack for as long as she is attacking the turret. I feel as if attacking a turret would stack her ranger's focus considering in the description of the ability, it says ranger's focus is stacked by attacking, and you are technically attacking the turret, therefore I believe it should stack while attacking the turret, "Ashe builds up Focus by attacking. At maximum Focus, Ashe can cast Ranger's Focus to consume all stacks of Focus, temporarily increasing her Attack Speed and transforming her basic attack into a powerful flurry attack for the duration."